An audio amplifier amplifies a low-power audio signal to drive a transducer, such as a speaker. The audio amplifier is an OP amplifier, whose driving ability can be adjusted. For example, when the audio amplifier drives a low-impedance apparatus, the driving ability of the OP amplifier should be increased in order to reduce its total harmonic distortion (THD).
When the audio amplifier is at a transient state of power on/off, since charging and discharging rates of capacitors in the circuit are various, unpleasant pop noise often occurs. A conventional audio amplifier is designed with consideration of an impedance range for driving a speaker. Driving abilities for a low-impedance speaker and a high-impedance speaker are different. As the driving ability of the OP amplifier increases, the pop noise becomes more serious. Conversely, as the driving ability of the OP amplifier decreases, the high-impedance speaker cannot be driven properly so that the THD gets greater. In other words, by merely adjusting the driving ability of the OP amplifier in the audio amplifier, problems of the THD and the pop noise of the audio amplifier cannot be simultaneously solved. The pop noise gets worse as the THD becomes smaller, whereas the THD becomes greater when the pop noise is reduced.
Since the conventional audio amplifier cannot reduce the THD and remove the pop noise simultaneously, a novel audio circuit or controller is urgently needed for achieving both objects of de-popping and reducing the THD.